1. Field of the Invention
The present invention relates to a switching circuit and an individual voltmeter. More specifically, this invention relates to a switching circuit which connects both ends of a unit cell connected in series with other unit cell to a capacitor sequentially and relates to an individual voltmeter apparatus having the switching circuit.
2. Description of the Related Art
As a conventional switching circuit, a flying capacitor circuit as shown in FIG. 3 is proposed (for example, JP,H11-248755,A and JP,2002-156392,A).
The flying capacitor circuit is provided with a capacitor and a plurality of selector switches S1 to Sn+1 which connect both ends of each unit cell among V1 to Vn to both ends of the capacitor sequentially, for a plurality of unit cells V1 to Vn which are connected in series. In FIG. 3, each unit cell of V1 to Vn constitutes a battery.
The (n+1) selector switches S1 to Sn+1 are provided for n unit cells V1 to Vn. A negative side of the unit cell V1 and a positive side of the unit cell V2 connected to the negative side of the unit cell V1 are connected to the capacitor C through a common relay switch S2. As shown in FIG. 4, it is also proposed a flying capacitor circuit that both ends of each unit cell of V1 to Vn are provided with two selector switches S1 to S2n. See, JP,H11-248757,A.
The flying capacitor circuit is used, for example, for voltmeter apparatus. A voltage of each unit cell is detected by measuring the voltage of the capacitor C when both ends of each unit cell of V1 to Vn are connected by switches.
A relay switch has been used conventionally for the selector switch. However, the relay switch is inferior in respect of cost, size, durability, response speed, etc. compared with semiconductor switches, such as FET. Thus, a semiconductor switch may be used for the selector switch.
The semiconductor switch, which is connected to a higher rank unit cell in the flying capacitor circuit, has a higher source potential. Accordingly, it is necessary to apply a high gate voltage to the gate of a semiconductor switch which is connected to a higher rank unit cell. Therefore, when the gate voltage is supplied by a power supply of a set of batteries, the gate voltage of the semiconductor switch connected to the higher rank unit cell is applied by a power supply between the higher rank unit cell and the lowest unit cell V1.
Referring to FIG. 3, each selector switch of S1, S2, . . . and Sn needs a gate voltage for a power supply of a unit cell V1, between V2 and V1, . . . , and between Vn and V1, respectively.
Then, a lower unit cell consumes more current to generate a gate voltage and it causes a problem of a variation of electric power consumption in each unit cell.